Membrane physiology

• cell membrane is 8-10nm thick.
• it is a lipid bilayer in which protein molecules are embedded
• bilayer of phospholipid molecules that are amphipathic, i.e. they consist of a polar,hydrophilic (water-loving) head and a non-polar, hydrophobic (water-hating) tail.

Components of cell membrane
1)Lipid component consists of four phopholipids:phosphatidylcholine (lecithin), phosphatidylethanolamine (cephalin),sphingomyelin and phosphatidylserine.cholesterol and glycolipids are also present.

• lipids that constitute bilayer are amphiphilic because of their hydrophobic (nonpolar) chains directed toward the center of the membrane and their hydrophilic (charged) heads directed outward.

• lipid components exhibit asymmetry in which  phosphatidylcholine (lecithin) and  sphingomyelin are located in outer leaflet; phosphatidylserine and  phosphatidylethanolamine (cephalin) are located in inner leaflet.
• cholesterol molecules are intercalated among the phospholipids of the membrane with its hydroxyl group at aqueous interface and remainder of the molecule within the leaflet.

2)Protein component consists of integral and peripheral protein

• peripheral proteins exhibit a looser association with membrane surfaces and can be easily disassociated from the lipid bilayer by changes in ionic strength and ph.

• Integral proteins are directly incorporated within the lipid bilayer.transmembrane proteins are integral proteinsthat span the lipid bilayer,exposing the protein to both extracellular space and cytoplasm.Many transmembrane proteins are also known as receptor proteins.

Receptor Proteins
a)Ion channel linked receptors:they include voltage gated ion channels,mechanical gated ion channels,and neurotransmitter gated ion channel.
b)G protein linked receptors:theses channels work through cAMP pathway or Calcium ion pathway
c)Enzyme linked receptors.

Intercellular Connections:

1)Tight junctions:

• are the attachment between the cell
• may be an intrercellular pathway for solutes,depending on the size, charge,and characteristics of the tight junction.
• may be Tight(impermeable),as in the renal distal tube,or Leaky(permeable), as in the renal proximal tubule and gallbladder.

2)Gap junction:

• are the attachment between the cell that permit the intercellular communication.e.g permit current flow and electrical coupling between myocardial cell.